1. Field of the Invention
The present invention relates to an image taking apparatus having the function of focusing an objective lens and the function of observing an object image formed by the objective lens and a lens apparatus mounted on the image taking apparatus.
2. Description of the Related Art
When an object is observed through an optical view finder (OVF) of a single-lens reflex camera, which is one of optical apparatuses, a light flux emitted from the objective lens is reflected by a reflecting mirror provided on an optical path behind the objective lens (image plane side) to change its optical path and introduced into an optical view finder including a pentaprism, etc. This makes it possible to see an object image formed by the light flux which has passed through the objective lens as a normal image. At this time, the reflecting mirror is placed diagonally on the image-taking optical path.
On the other hand, when the objective lens is used as an image-taking lens (when an image is taken), the reflecting mirror is instantaneously withdrawn from the image-taking optical path and an image-taking light flux which has passed through the objective lens is formed on an image pickup medium (film and image pickup element such as CCD). Then, when the image-taking is completed, the reflecting mirror is instantaneously placed diagonally on the image-taking optical path.
In a digital camera, in response to a release button depressing operation, an object image is exposed to light for a desired time on an image pickup element such as a CCD or CMOS sensor and an image signal expressing one still image obtained through photoelectrical conversion of the image pickup element is converted to a digital signal. Then, by applying predetermined processing such as YC processing to the converted digital signal, an image signal of a predetermined format is obtained.
The digital image signal expressing the image picked up is recorded in a semiconductor memory for each image. The recorded image signal is read out from the memory and displayed on a display unit of the camera, reproduced as a printable signal or output to and displayed on a display device, etc.
One type of the above described camera is a single-lens reflex type digital camera which allows manual selection of focusing operation according to a phase difference detection system or focusing operation according to a contrast detection system as disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-275033 (hereinafter referred to as “Document 1”).
Furthermore, in a single-lens reflex type digital camera provided with an optical view finder and electric view finder (EVF) as disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-125173 (hereinafter referred to as “Document 2”), when the reflecting mirror is placed diagonally on the image-taking optical path, focus detection according to a phase difference detection system is performed based on the light flux reflected by the reflecting mirror and when the reflecting mirror is withdrawn from the image-taking optical path, focus detection according to a contrast detection system is performed using the output of an image pickup element which receives the image-taking light flux.
In the camera disclosed in above Document 2, it is possible to electronically display the image which has been taken while performing focusing based on the output of the image pickup element even when the reflecting mirror is in a position withdrawn from the image-taking optical path. Therefore, the photographer can take images while checking the focusing state of the image which is electronically displayed on, for example, an organic EL display.
On the other hand, in focusing control, to increase the speed of deciding the focusing direction (driving direction of image-taking lens), there is a focusing apparatus provided with a step in a light detecting surface of the image pickup element as disclosed, for example, in Japanese Patent Application Laid-Open No. 2001-215406 (hereinafter referred to as “Document 3”). That is, the optical path length is differentiated by a micro distance, a plurality of image signals are collected and the focusing direction is decided based on the collected image signal. Then, the image-taking lens is moved to the in-focus position in the decided focusing direction.
Furthermore, as disclosed, for example, in Japanese Patent Application Laid-Open No. 2000-162494 (hereinafter referred to as “Document 4”), there is a camera system provided with a focus detection units according to a phase difference detection system in each of a lens apparatus and a camera body. The method of detecting a focusing state according to a phase difference detection system is disclosed, for example, in Patent Publication No. H5 (1993)-88445 (hereinafter referred to as “Document 5”).
The camera disclosed in above described Documents 1 and 2 performs focusing according to the contrast detection system, but this detection system has a problem that it takes time to reach an in-focus state.
The focusing according to the contrast detection system calculates sharpness of an object image formed by the image-taking optical system through an evaluation using a predetermined function based on the output of the image pickup element and adjusts the position on the optical axis of the image-taking optical system in such a way that the function value takes an extreme value. There are various evaluation functions such as adding up absolute values of differences in brightness signals of adjacent pixels within the focus detection area, adding up the squares of differences in brightness signals of adjacent pixels within the focus detection area or likewise processing differences in signals of adjacent pixels of R, G and B image signals.
The focusing according to the contrast detection system calculates an evaluation function value while slightly changing the position on the optical axis of the image pickup optical system (focusing lens), and thereby takes a considerable time until an in-focus state is achieved.
On the other hand, the focusing apparatus disclosed in above described Document 3, the speed of the focusing operation is enhanced, but pixels of a short optical path length and pixels of a long optical path length are mixed, which makes it not possible to obtain a high quality image. Here, shortening difference of the optical path length between pixels of a short optical path length and pixels of a long optical path length makes it possible to improve the image quality but it is difficult to decide focusing the direction in focusing control. Therefore, the above described focusing apparatus cannot realize high-speed focusing control and improvement of image quality simultaneously.
The camera system disclosed in above described Document 4 is provided with a focus detection unit according to a phase difference detection system, and can thereby recognize a defocus amount through a single focus detection operation. For this reason, if a focusing lens of the objective lens unit is driven based on the defocus amount, one-time lens driving is all that is required to reach an in-focus state allowing extremely high speed focusing.
Furthermore, in the above described camera system, the reflecting mirror is kept withdrawn from the image-taking optical path during continuous image-taking, the focal plane shutter is fully open and the focus detection unit in the lens apparatus performs a focus detection operation. Therefore, even if the reflecting mirror is withdrawn and the focus detection unit in the camera body is not operating, it is possible to perform high-speed focusing based on the focus detection by the focus detection unit in the lens apparatus.
However, since the above described camera system is provided with two focus detection units, the system becomes a large and costly system.